I installed CameraRaw 9.0 as an option, but that does not seem to have done the trick either?

Just for information - ACR has no relevance whatsoever on whether Lr will open a file.

The D750 has been supported since November 2014 (Lr 5.7) so you're either using an older version of Lr, or - depending on what actuallyhappens when Lr "doesn't work" - this might be (as Steve mentions above) down to something like the files having been touched by Nikon software on import (which does something to the file headers that renders them unreadable by Lr); or a problem with the Write permissions on your machine.

So - some actual details of what you're seeing would be helpful, along with your OS.